1. Field of the Invention
The present application relates generally to structural building components and more particularly, but not by way of limitation, to methods and systems for thermal insulation of structural building members to reduce heat transfer.
2. History of the Related Art
The trend of increasing prices for natural gas, electricity, and other heating fuels have made energy efficiency a high-profile issue. In buildings, thermal energy may be lost to the atmosphere through, for example, conduction, radiation, or convection. Radiation is a transfer of thermal energy through electromagnetic waves. Convection takes place as a result of molecular movement, known as currents or convective looping, within fluids. A common mode of convection occurs as a result of an inverse relationship between a fluid's density and temperature. Typically, such type of convection is also referred to as “natural” or “free” convection in which heating of a fluid results in a decrease in the fluid's density. Denser portions of the fluid fall while less dense portions of the fluid rise thereby resulting in bulk fluid movement. A common example of natural convection is a pot of boiling water in which hot (and less dense) water at a bottom of the pot rises in plumes and cooler (more dense) water near the top of the pot sinks. The primary means of thermal energy loss across an un-insulated space is natural convection.
Conduction is a transfer of thermal energy between regions of matter due to a temperature gradient. Heat is transferred by conduction when adjacent atoms vibrate against one another. Conduction is the most significant form of heat transfer within a solid or between solid objects in thermal contact. Conduction is more pronounced in solids due to a network of relatively fixed spatial relationships between atoms. Conductivity varies with a material's density. Metals such as, for example, copper and aluminum, are typically the best conductors of thermal energy.
Thermal efficiency of building components are often expressed in terms of thermal resistance (“R-value”) and thermal transmission (“U-factor”). R-value is a measurement of thermal conductivity and measures a product's resistance to heat loss. In common usage, R-value is used to rate building materials such as, for example, insulation, walls, ceilings, and roofs that generally do not transfer significant amounts of heat by convection or radiation. A product with a higher R-value is considered more energy efficient.
Of particular concern in buildings are windows and doors. In particular, windows come in contact with the environment in ways that walls and solid insulation do not. As a result, windows are strongly affected by convection as well as radiation. For this reason, U-factor is commonly used as a measure of energy efficiency of windows. U-factor measures a rate of total heat transfer through a product (including heat transfer by convection and radiation). A product with a lower U-factor is considered more energy efficient. In recent years, federal, state, and municipal building codes often specify minimum R-values and U-factors for building components.